Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. As the traffic on fiber optic networks increases, monitoring and management of the networks become increasingly more significant issues. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.” In this document, these individual information-carrying lights of a WDM optical fiber, optical line or optical system are referred to as either “signals” or “channels” and the totality of multiple combined signals, wherein each signal is of a different wavelength range, is referred to as a “composite optical signal.” To monitor the network, the spectral characteristics of the composite signal at particular points in the network must be determined and analyzed. This information may then be used to alter the performance of the network if the signal characteristics are less than optimal.
FIG. 1 illustrates a conventional system for monitoring the characteristics of a plurality of composite optical signals, wherein each composite optical signal is transmitted along a respective fiber-optic line from among a plurality of fiber-optic lines. The conventional system 100 shown in FIG. 1 comprises a set of fiber optic lines 112a-112e, a set of Optical Performance Monitors (OPM's) 102a-102e and a set of optical taps 110a-110e. Each optical tap is optically coupled to a respective one of the fiber optic lines and splits off a small sampled proportion (typically ca. 1-2%) of the composite optical signal carried by said line to a respective one of the OPM's. For instance, a sample of a first composite optical signal (COS) transmitted along fiber optic line 112a is split off by optical tap 110a to the OPM 102a. Similarly, a separate sample of a different COS transmitted along fiber optic line 112b is split off by optical tap 110b to the OPM 102b, etc.
Each one of the OPM's 102a, 102b, etc. comprising the conventional system 100 continuously measures important spectral characteristics of the sampled COS delivered to it. Such spectral characteristics may include the number of active channels comprising each COS, the absolute and relative intensities of the channels and the wavelengths of the channels. Since each OPM is optically coupled to one and only one fiber optic line, via a respective optical tap, each OPM is dedicated to monitoring the spectral parameters of a particular fiber optic line and there are as many OPM's as there are fiber optic lines.
Although the conventional optical performance monitoring system 100 can adequately perform its intended functions, the dedication of each OPM to a single fiber optic line add a significant degree of cost to the installation and operation of a fiber optic network. Optical performance monitors are typically complex and relatively expensive instruments, comprising finely adjusted wavelength dispersion elements such as diffraction gratings as well as a plurality of photo-detectors, associated control electronics and software. The level of OPM duplication provided in the conventional system 100 multiplies the costs required to set up the network. Further, each OPM may, periodically require re-calibration or adjustment. Thus, maintenance costs are likewise multiplied.
Accordingly, there exists a need for method and system for an optical performance monitor that is constructed with less duplication of apparatus than is the conventional system and that can operate more cost-efficiently than the conventional system. The present invention addresses such a need.